1. Field of the Invention
The present invention relates to a method for the inspection of a circuit board, particularly to a method for inspecting a circuit board having a number of pattern electrodes by inspecting the state of electrical connection of the electrodes.
2. Discussion of the Background
In order to inspect the electrical characteristics of a circuit board in which a number of electrodes are formed, it is generally necessary to contact the electrodes to be inspected of the circuit board with the inspection electrodes of a tester having the corresponding pattern arrangement as the electrodes to be inspected, thereby achieving electrical connection between the two. Hence, conventionally, alignment between the circuit board and the inspection electrodes portion of the tester has been conducted by, for example, a means of fitting the alignment pins of the tester into the corresponding pin holes of the circuit board, or other mechanical alignment means.
In recent years, the parts packing density of a circuit board has been increased and the density of pattern has become higher. As a result, it is being pushed forward to make the electrodes of the circuit board smaller and the connection lines of the circuit board finer and arrange them in a higher density. In inspecting such a circuit board, when the alignment between the circuit board and the inspection electrodes portion of a tester is effected by a simple mechanical alignment means, merely a rough alignment state is obtained and it is impossible to sufficiently achieve the desired electrical connection between the electrodes to be inspected and the corresponding inspection electrodes.
The reasons therefor are that even when the electrodes to be inspected of the circuit board and the inspection electrodes of the tester have basically the corresponding patterns, it follows that the electrodes of the circuit board or the inspection electrodes are actually located at positions deviated, though slightly, from the design positions for the reasons associated with the production of the circuit board and tester or owing to the deformation of the circuit board and tester themselves.
Under such circumstances, it is necessary to make rough alignment between the circuit board to be inspected and the inspection electrodes portion of the tester by, for example, a mechanical alignment means and then make fine alignment to achieve precise position adjustment of the electrodes to be inspected of the circuit board and the inspection electrodes of the tester.
For achieving such fine alignment, a method has been known which comprises forming an appropriate alignment mark on a circuit board in a specific position relation to the electrodes to be inspected; forming a control alignment mark on the inspection electrodes portion of a tester in the same relation to the inspection electrodes; fixing the circuit board onto a movable stage capable of moving the circuit board in the lengthwise or crosswise direction or about the rotating axis; mounting them on the tester and checking the positions of the two alignment marks by an appropriate detection means; based on the detection result obtained, displacing the circuit board relatively to the inspection electrodes portion of the tester via the movable stage to obtain a state in which the two alignment marks coincide in position with each other.
Such a fine alignment means, however, requires that, for example, an alignment mark-observing camera or other alignment mark-detecting means are provided in order to detect the alignment marks formed on the circuit board and the inspection electrodes portion. Moreover, such an alignment mark-detecting means must be arranged above the alignment mark and in a direction vertical to the circuit board, and, as a matter of course, the inspection electrodes portion of the tester must be arranged so that the surface of the circuit board is covered therewith; hence, the alignment mark-detecting means and the inspection electrodes portion of the tester must be provided at the same place, which makes it difficult to provide them each at an appropriate place.
Further, with respect to the alignment marks per se formed on the circuit board and the inspection electrodes portion of the tester, it is impossible to completely prevent errors in position and form from being caused in the formation of said marks. Moreover, an error is caused to a certain extent in the detection per se of the degree of position-adjustment of the two alignment marks. Consequently, a complicated procedure is necessary until the desired fine alignment is achieved satisfactorily, and the achievement of fine alignment is not fully reliable in respect of repeatability, etc.
As described above, all of the conventional fine alignment methods aim at alignment between the electrodes to be tested of a circuit board and the inspection electrodes of a tester, but it is not always possible to achieve alignment with high reliability because each of the methods uses an indirect deviation-detecting means placed in a specific position relation to the circuit board or the inspection electrodes portion of the tester.